Generally, in a vehicle obstacle detection device using a radar, it is extremely important to discriminate between a target vehicle (another vehicle around an own vehicle) as a target object to be detected, and an object other than the target vehicle. An error in the discrimination leads to a situation where an object other than the target vehicle is recognized as the target vehicle, and consequently becomes a factor for erroneous alarm or erroneous control.
For example, a vehicle obstacle detection device for detecting, as an obstacle, another vehicle around an own vehicle is configured to attach a radar unit to a rear end panel in front of a rear bumper, through a bracket, and transmit a radio wave from the radar unit toward an outside of the own vehicle through the resin bumper, thereby detecting the another vehicle. However, if, as a result, for example, of undergoing reflection by a back surface of the rear bumper, a part of a transmission wave from the radar unit is oriented to pass between a transmitter section of the radar unit and the back surface of the rear bumper and reach a rear wheel of the own vehicle, and then a reflected wave returned from the rear wheel is input into a receiver section of the radar unit, a problem occurs that the rear wheel of the own vehicle is misdetected as a target vehicle although it is not really a target vehicle.
As measures to prevent such a misdetection, it is conceivable to arrange a transmitter-receiver section of the radar unit in close contact relation with the back surface of the bumper. However, this is undesirable from a practical standpoint, because an external impact force added to the bumper due to a mild rear impact collision or the like leads immediately to malfunction and/or damage of the radar.
Meanwhile, the following Patent Document 1 discloses a radar unit designed to reduce a reflection loss due to a back surface of a bumper, wherein a reference frequency fo of a transmission beam is controlled based on a reception beam, to minimize the reflection loss from the bumper. However, the Patent Document 1 neither discloses nor suggests the aforementioned misdetection.
The following Patent Document 2 discloses a radio wave radar mounting structure in which a shield plate or a radio wave absorbing material is provided between a radio wave radar and a radiator fan located behind the radio wave radar to prevent a situation where a side lobe emitted from the radio wave radar and reflected by a vehicle body reaches the radiator fan. Even if the radiator fan is located behind the radar, this structure makes to possible to prevent a rise in noise floor due to detection of the radiator fan by the side lobe, thereby obtaining stable target detection performance. However, the Patent Document 2 does not disclose a technical problem of how to prevent the misdetection caused by the phenomenon that a part of a transmission wave from a radar unit passes between a transmitter section of the radar unit and a back surface of a bumper and reaches a wheel of an own vehicle, and then a reflected wave is returned from the wheel.